I have had my 1987 Camaro for 3 years now. When I first got it the left rear caliper was sticking on occasion. When I would drive away after it sat overnight the left rear brake would groan then seem to free up after a few feet. I had the caliper replaced and it was better, but would still do it now and then.
I just got the car out of winter storage, (sat for 5 months) and when I drove ahead it was really groaning, worse than ever. It also did it in reverse. Took a while for it to free up.

How could it have the same symptom with 2 different calipers but on the same location? What to look for and what to check?

I will add that I do not use the parking brake.

 

Might want to replace your brake hoses. They can start to degrade internally and act as a check valve keeping hydraulic pressure on the caliper.

I'd replace all soft hoses and flush the brake fluid.

 

When one caliper sticks, it could be a binding caliper or a bad hose, , , but when it's replacement also sticks, DaveyDug nailed it, you need new hoses.....

 

If one is bad I would replace all three hoses back there.

 

​​​​​​​

 

An 87 model with the original cast iron calipers doesn't have a brake hose on the rear left caliper. Just on the right side and in the middle from frame to the rear axle.

 

Those brakes are garbage. Update to 89-97 rear disc at least